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Superfluidity
Superfluid
Superfluidity is a
state of matter characterised by the complete absence of
viscosity. Thus superfluids, placed in a closed loop, can flow endlessly
without
friction. Superfludity was discovered by
Pyotr Leonidovich Kapitsa,
John F. Allen, and
Don Misener in 1937.
The superfluid transition is displayed by
quantum liquids below a characteristic transition temperature. The most
abundant isotope of
Helium,
4He, becomes superfluid at temperatures below 2.17K
(-270.98°C).
The less abundant isotope,
3He, becomes superfluid at a much lower temperature: 2.6mK
(only a few thousandths of a degree above the absolute zero, that is -273.15°C).
Although the phenomenology of superfluidity in these two
systems is very similar, the nature of the two superfluid transitions is very
different.
4He atoms are
bosons, and their superfluidity can be understood in terms of the
Bose statistics that they obey. Specifically, the superfluidity of
4He can be regarded as the generalisation of
Bose-Einstein condensation (which takes place only in a non-interacting gas)
to interacting systems. On the other hand,
3He atoms are
fermions, and the superfluid transition in this system is described by a
generalisation of the
BCS theory of
superconductivity. In it,
Cooper pairing takes place between atoms rather than electrons, and the
attractive interaction between them is mediated by spin fluctuations rather than
phonons. A unified description of superconductivity and superfluidity is
possible in terms of
Gauge symmetry breaking.
One important application of superfluidity is in
dilution refrigerators.
The study of superfluidity is
quantum hydrodynamics.
Recently in the field of chemistry, superfluid helium-4 has
been successfully used in spectroscopic techniques, as a
quantum solvent. Referred to as
Superfluid Helium Droplet Spectroscopy (SHeDS), it's of great interest in
studies of gas molecules, as a single molecule solvated in a superfluid medium
allows a molecule to have effective rotational freedom - allowing it to behave
exactly as it would in the gas
phase.
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